Traditional blow molding is limited as to the wall thickness of the article to be formed, as well as the complexity of article shape. To overcome this, thermoforming, a modification of blow molding, can suffice for manufacturing articles having relatively thick walls and/or complex shapes. Thermoforming processes such as plug assisted vacuum forming or pressure forming permit the production of items having a wall thickness of up to about ⅜ inch (95.25 mm). Articles formed by conventional blow molding, by contrast, are usually limited to wall thicknesses of less than about ⅛ inch (31.75 mm). This is due, in part, to the negative effects exerted on the blowing process by the greater volumes of polymer resin required to achieve thicker walls. For example, increasing amounts of viscous molten polymer will limit the size, wall thickness and complexity of an article to be formed, as blown air becomes progressively ineffective at expanding molten polymer as the volume of polymer material increases.
In basic vacuum forming, a carrier frame delivers a heated plastic sheet to a mold assembly, after which the sheet is clamped and sealed against the mold edge surfaces. Application of a vacuum causes atmospheric pressure to force the sheet against the mold cavity to assume the cavity shape. Mold cooling promotes the formation of a thin sheet having the dimensions defined by the mold.
As a variation of blow molding, the above-mentioned process further includes the step of blowing air of controlled pressure to force the heated sheet away from the cavity into a bubble. A shaped plug is then inserted into the bubble, pressing the bubble back into the mold cavity after the sheet has been sealed across the mold cavity. Upon reaching the bottom of the mold cavity, compressed air and/or a vacuum is applied to force the sheet against the mold. After forcing the sheet into the cavity, a full vacuum is applied from the cavity side and positive pressure is applied from the plug side of the apparatus to complete the formation of a molded article. After it has solidified, the mold assembly is opened, and the article is removed.
In a similar fashion, drape forming entails either draping a plastic sheet over or moving a male mold into a plastic sheet, and thereafter clamping, heating, and sealing the sheet over the male mold. Numerous vent holes in the mold apparatus permit a vacuum to be drawn, allowing atmospheric pressure to force the draped sheet into the contours of the mold cavity. Upon cooling, the sheet shrinks onto the mold.
Typical vacuum-formed or pressure-formed products include blister and skin packaging, food and drink containers, toys, luggage, and auto and appliance parts. Polystyrene, polypropylene, HDPE, thermoplastic polyester, ABS and vinyls are often used to manufacture these articles. Films and sheets formed in this fashion are often laminated by melt or adhesive processes to enhance their functional performance.
A need has arisen for reinforced blow molded articles having good thermoinsulating and sound barrier properties. In particular, the resurgence in popularity of removable hard tops and T-tops for automobiles has prompted engineers to seek better insulating characteristics of blow molded articles. For example, lightweight, suitably thermoinsulated removable hard tops for sport utility vehicles (SUVs) are in high demand by consumers. While blow molding provides for sufficiently lightweight automobile parts, combining the suitable weight properties with good impact resistance and thermoinsulating properties has heretofore been difficult.
The usefulness of blow molding techniques for forming such impact resistant, thermoinsulated articles has not been practical due to the structural characteristics of the plastic material conventionally used in blow molding. That is, the ability to blow mold light weight, thermoinsulated parts is limited by the fact that the parts produced can be only so large or so thin before the parts lose their structural integrity and impact resistance.
Further, most insulating materials must be laminated to the part after blow molding into the desired shaped. For example, urethane foam may be introduced to a blow molded part to improve insulating capabilities, as well as dimensional stability. However, this process is plagued by incompatibility between the skin component of the molded part and the insulating foam filler. Expensive thermoplastic skins are often chemically incompatible with traditional foam insulating materials, preventing strong bond formation within laminated structures. Thus, blow molded articles having skin and foam fillers of different materials are prone to delamination. A solution to the delamination problem is to fill the article with a foamed resin identical to the resin used to form the exterior skin of the article. Although this expensive concept is acceptable for many blown articles, it is insufficient for producing a cost effective automobile part having good impact resistance.
Blow molded articles such as sport utility vehicle (SUV) hard tops require good thermoinsulation while exhibiting strong impact resistance. By nature, structural foams lack good impact resistance due to their open cellular conformation. Thus, blow molded automobile parts having structural foam insulating materials compatible with an exterior resin skin require reinforcement.
Heretofore, in order to reinforce various plastic parts, such parts would conventionally comprise resins fortified by mineral fillers or glass fibers. However, such reinforcement cannot be used effectively in blow molding operations, because the glass fibers limit parison expansion characteristics and also have a deleterious effect on the blow molding assembly itself. Furthermore, such reinforcement has a deteriorating effect on the foaming capabilities of resins. Thus, blow molded articles having a structural foam component subjected to conventional reinforcement often lack uniform strength and impact resistance.
Similarly, thermoformed articles having foam backing typically lack satisfactory levels of impact resistance due to both the need for an aesthetically pleasing skin and the open cellular nature of reticulated foam. Exterior skin appearance deteriorates with increasing amounts of conventional reinforcing materials. Typical reinforcing materials tend to impair the formation of reticulated cells during blowing of foam resins. Because structural foams are not adequately reinforced by conventional means, thermoformed articles comprising good quality skins laminated to foam backing have inadequate strength and impact resistance.